1. Field of the Invention
The invention concerns a mobile working device, in particular, a concrete pump, with a chassis, with two front and two rear supporting extension arms which can be extended from a travel position into at least one support position and with respectively one telescopic support leg which can be supported upon a foundation and with respectively one measuring member for determining the support load acting upon the support legs.
Mobile working devices of this type are provided with extendable support legs, which are intended to improve the stability of the work device at the site of employment. On the one hand, the support legs have the task of eliminating the spring travel and relieving the loads on the wheels. On the other hand, the supporting extension arms have the task of minimizing the danger of tipping, which occurs when a working extension member (boom) produces high overturn moments. The support legs of the supporting extension arms form the corners of a rectangle, of which the side lines describe an area, within which the center of gravity of the working device must lie, in order to ensure a stable stance. Since the projecting working extension member (boom) is rotateable, the center of gravity during rotation describes a complete circle, wherein the working zone of the working extension member must lie within the rectangle. Since the space requirement in a construction site is limited, frequently a complete extension is not possible. As a result the pivot range of the working extension member (boom) is limited.
2. Description of Related Art
In order to ensure safety against tipping, a monitoring device has already been described (the journal “Concrete” 6/96, pages 362, 364). There, the pressures existing in the four hydraulic actuated telescopes of the support legs are monitored. If the pressure drops in two support leg cylinders, then the boom movement and the concrete pump are switched off. This technique can also be used in the case that a machine, for reasons of restricted space, is not completely or fully supported. Research has shown that pressure measurement in the telescopic cylinders of the support legs is not a sufficient basis for a reliable monitoring of stability. This applies above all in the case when one of the telescopic cylinders is fully extended. Further, dynamic support effects cannot be detected by the monitoring systems.
In order to overcome these disadvantages, it has already been proposed (DE-A10110176), that in the foot part of each support leg a pair of force sensors is provided. For this, individual specially constructed support feet are necessary, of which the construction cost is quite high. Further, herein each force sensor is provided with its own electrical measurement circuit for providing a support load dependent measurement signal, wherein the monitoring device includes an evaluation unit, which in predetermined sampling times receives support load measurement values relating to the support feet and, for comparison, at least one predetermined stability determining threshold value. The evaluation unit includes a software routine for determining the second lowest support foot related support load measured value for each sample cycle and for the comparison thereof with a stability determining threshold value. A disadvantage herein is that the threshold value is a construction-dependent, dimensionally-contingent value, and that for the determination of the stability relevant load value a high measurement precision is necessary even in the case of low loads.